Recently, an increasing number of vehicles are equipped with various driving assistance capabilities, examples of which include automatic lane keeping control, automatic braking control, and adaptive auto-cruise control that have already been installed in a lot of vehicles on the market. For example, Patent Document 1 discloses a driving assistance technique for relaxing the driver's attention to the surrounding environment in order to relieve his or her tension. Patent Document 2 discloses a driving assistance technique for alleviating the driver's muscle tone during driving with the intervention of a control program.